This invention is directed to the combination of a pair of aligned tubular members joined by an internal expansion coupling device that offers improved thrust restraint, particularly for plastic pipe, such as polyethylene, PVC, fiberglass and fiberglass reinforced polyester (FRP). Thrust restraint, as used herein, relates to the problem associated with the relative movement, i.e. axial separation, of coupled pipes or tubular members, and the loss of seal or effectiveness of the joint.
It was discovered by one of the inventors hereof, as exemplified by U.S. Pat. Nos. 4,927,189 and 4,997,212, that tight and effective seals can be achieved by the use of an internal expansion coupling device, as taught therein, especially in relatively static situations. Such device comprises an elongated open ended tubular joint member formed of a relatively rigid sheet-like member, where the ends are turned upon each other in overlapping relationship. A continuous sleeve, formed of an elastomeric material, is provided to overlap the sheet side, where the sleeve, about the circumference thereof, may be provided with at least one continuous upstanding rib which is adapted to lie adjacent to and be compressed against the inside wall of a tubular member. Finally, means for expanding the sheet-like member is provided, whereby the sheet ends move circumferentially towards one another urging the elastomeric sleeve and rib against the inside wall of a tubular member. Such a device has proved successful in effecting a tight seal between the tubular members.
However, there is a strong need, particularly in the plastic transmission line industry, for an effective coupling system that offers the advantages of ease of assembly and disassembly, tight seal to the flow medium, and thrust restraint. By the nature of the product, particularly such plastics as polyethylene, PVC, fiberglass and FRP, it has been necessary to follow a very labor intensive and time consuming practice to join plastic pipe. A typical joint for plastic pipe, laid end-to-end, is to apply an external sleeve or fitting socket to the pipe ends, where the sleeve is provided with an internal continuous shoulder or pipe stop for the pipes inserted therein. Since no mechanical devices are used, the sealing thereof has to rely on adhesives. For instance, using PVC as an exemplary pipe material, the surface end portions of the pipes are subjected to a cleaner-primer, the purpose of which is to soften and dissolve the surface to be joined to the sleeve, followed by the application thereto of a solvent cement. Unfortunately, there are numerous precautions or practices which must be followed.
Pipe size, particularly large diameter pipe, is a major factor. For example, the basic solvent cement instructions apply to all sizes of pipe, but when making joints 4" and above, the use of two men to apply the solvent cement simultaneously to pipe and fitting is recommended. Additional men should also be in a position to help "push" the pipe into the fitting socket while the cemented surfaces are still wet and ready for insertion. Thus, labor costs have a significant impact on this traditional manner of joining plastic pipe. There are additional concerns. Alignment of large diameter pipe and fittings is much more critical than when working with small diameter pipe. As the pipe diameters increase, the range of tolerances also increases, which can result in "out of round" and "gap" conditions. Speed in making the joint and applications of heavy coats of solvent cement in these cases is important.
Temperature and atmospheric conditions are further factors to be considered. In cold weather conditions, the typical primers take considerably longer time to penetrate the surface of the plastic. Further, at cold temperatures, it is important to keep the primers and cement, and even the fittings warm. There are just as many problems or concerns under warm weather conditions.
To further complicate this cumbersome prior art practice, cure times will vary with pipe sizes, temperature and humidity conditions. As a practical matter, a complex table of cure times is required for any pipe laying operation. For instance, the cure time for 20" pipe is dry summer-like weather in about 36 hours. The same pipe, under humid but freezing conditions, would require a cure time of from 20 to 24 days, a significant difference. After all this time and effort, and only then, may the coupled pipes be pressure tested. The results at best are good, which would certainly suggest some failures.
The present invention avoids such delays and labor costs with a coupling system that is effective, easily assembled and disassembled, and one that may be used or tested immediately. The coupling system of this invention will become clearer in the description which follows.